Quick answer: Yes and no. Any religious or doctrinal aspects of a kesubah itself cannot be enforced under American laws because of Constitutional issues involving the free exercise and establishment clauses to the First Amendment. However, courts have and can enforce strictly secular sections of kesubahs or separate secular agreements between a Jewish couple provided that the court is not asked to interpret Jewish law or do the work of the Jewish court. E.g. a court can't force a husband to give his wife a get. Nor can it enforce payment terms under a kesubah. But, many state courts have ruled that if courts can apply neutral areas of law to the issue, rather than intepreting religious doctrine, then courts have jurisdictin to, or example, enforce a pre-nuptual agreement that requires a husband and wife to seek mediation through a beis din, or force the husband to pay $100 per month in child support for each month he fails to appear before the beis din.
Background: Courts have interpreted the First Amendment broadly to severely circumscribe the role that civil courts may play in resolving disputes concerning issues of religious doctrine and practice. E.g. DeCorso v. Watchtower Bible & Tract Society of New York, Inc., 829 A.2d 38 (Conn. App.), cert. denied, 837 A.2d 805 (Conn. 2003). By contrast, exercise of governmental authority is permissible if it (1) has a secular purpose, (2) neither inhibits nor advances religion as its primary effect and (3) does not create excessive entanglement between church and state. Lemon v. Kurtzman, 403 U.S. 602, 612-13, 91 S.Ct. 2105, 29 L.Ed.2d 745 (1971).
Freedom of religion is guaranteed not only to individuals but also to churches, and church organizations, which have “power to decide for themselves, free from state interference, matters of church government as well as those of faith and doctrine.” Kedroff v. St. Nicholas Cathedral, 344 U.S. 94, 116 (1952).
Connecticut and New Jersey courts have found that they could enforce a provision under Moslem law where couples in Iran had entered into prenuptual agreements calling for the husband to give the wife a mahr -- a large amount of money -- at the wedding, and additional sums during the marriage. Since no money was paid at either times, the wives in both cases sued for the promised funds after the divorce, and the courts found they could resolve the issues using neutral legal principles. Lashgari v. Lashgari, 496 A.2d 491 (Conn. 1985); Odatalla v. Odatalla, 810 A.2d 93 (N.J.Super. 2002).
In 1926, a New York Appellate Division court upheld the trial court's jurisdiction to enforce a kesubah's provision that the wife could not be evicted from her home by her step-children following the death of her husband. Hurwitz v. Hurwitz, 216 A.D. 362 (N.Y. App. Div. 2d 1926). Also, the case of Avitzur v. Avitzur, 446 N.E .2d 136 (N.Y.), cert. denied, 464 U.S. 817, 104 S.Ct. 76 (1983), the court held that it had jurisdiction to rule on secular sections of a kesubah.
Since then, the Rabbinical Council of America and other rabbinic organizations have urged couples to enter into pre-nuptual agreements which require, in the event the couple files for divorce in secular court, that they agree to mandatory arbitration before an Orthodox Jewish beis din (which may require the husband to provide his wife with a get (Jewish divorce decree). Such contracts have been upheld in Connecticut, Illinois, California, Georgia, Maryland and Colorado. Cites available upon request.